1. Field of Invention
This invention relates to a diperoxyester mixture composition of at least two symmetrical and at least one unsymmetrical diperoxyester. This invention also comprehends processes to prepare and use such a composition.
2. Description of the Prior Art
Attempts have been made in the prior art to increase the efficiency of polymerizing polyvinyl chloride and polystyrene by using a more active free-radical initiator. One approach is to use a two stage styrene suspension polymerization process using a combination of benzoyl peroxide and tertiary butyl perbenzoate as the initiator system as described in U.S. Pat. No. 2,656,334. The overall polymerization time is 9 to 10 hours. A three stage polymerization is described in U.S. Pat. No. 2,907,756 wherein the rate of radical generation is kept constant throughout the process resulting in a reduction of polymerization time. This process also uses benzoyl peroxide as the low temperature initiator component. The use of an unsymmetrical diperoxyester for the polymerization of a vinyl monomer in two heating stages is described in U.S. Pat. No. 3,585,176 wherein a single diperoxyester containing two peroxy groups of different reactivities is used. This one component system decomposes into free-radicals at two different temperatures but the polymerization time is not decreased. Another approach is to use a three component initiator system under a continuously increasing programmed temperature cycle (British Pat. No. 1,243,197). The polymerization time is reduced by this system. The disadvantage of the above mentioned approaches is that none of them can reduce the polymerization time down to three or four hours as the present invention can.
Ivanchev, et al [Vysokomol. Soyed., All, (9), 2082 (1969), Al2, (2), 450 (1970)] investigated the use of symmetrical diperoxides and unsymmetrical diperoxides in isothermal styrene polymerization. They reported that the unsymmetrical diperoxides were capable of giving much higher molecular weight polymer but the molecular weight distribution was bimodal (i.e., two peaks observed when the intensity [concentration] of polymer fraction was plotted against polymer molecuar weight) at both low and high conversions. When symmetrical diperesters were used, Ivanchev et al. reported that the molecular weight distribution was unimodal (i.e., only one peak observed when intensity was plotted against polymer molecular weight), like the monoperesters but the maxima in the molecular weight distribution curve for the diperesters was lower than that obtained with monoperesters. (e.g., t-butyl perbenzoate). Hence, Ivanchev indicates that symmetrical diperoxides give a low molecular weight polymer while the unsymmetrical diperoxides give a high molecular weight polymer but with a molecular weight distribution that is bimodal, and in some instances trimodal.
For commercial polymers (e.g., crystal polystyrene), a unimodal molecular weight distribution is required in order that the processing characteristics remain unaffected. Thus the prior art suggests that unsymmetrical diperoxides alone will not give an acceptable unimodal molecular weight distribution.
The diperoxyester mixture composition of the present invention was observed to increase efficiency when used in vinyl polymerization by improving the quantity of polymer in a shorter reaction time (about 3 to 4 hours) than the prior art. The polymers prepared using the composition mixtures of the present invention have higher polymer molecular weight than similar polymers produced using unsymmetrical diperoxesters and conventional initiators of the prior art. The polymers also had a unimodal molecular weight distribution. This was unexpected according to molecular weight distribution of polymers produced from art unsymmetrical diperoxides.
The components of the mixture composition of the present invention are simultaneously prepared as a mixture in a simple process. An attempt to prepare this mixture composition from pure substances would be difficult. Making up a particular mixture would require at least three separate preparations of the pure symmetrical and unsymmetrical diperoxyester components. The preparation of the pure unsymmetrical diperoxyester component involves a plurality of complex steps. In a composition having more than one unsymmetrical diperoxyester component, the amount of time and work involved in preparing these pure diperoxyesters would be substantially multiplied.